The invention relates to electrode compositions for Li-ion rechargeable batteries, having a low porosity and low electrolyte: cathode material ratio, but with a large ionic transport rate. This results in a battery having improved safety and good rate performance.
Rechargeable lithium batteries have many advantages compared to other battery systems. They show high energy density, high voltage, absence of memory effect and good cycle stability. Currently two of the major drawbacks are problems related to ionic conductivity of cathode and electrolyte, and the lack of safety of the charged battery.
The ionic conductivity of cathode materials is low. Thus liquid electrolytes—which have much faster transport rates for lithium—are used. The electrolyte fills a network of interconnected pores, ranging from the cathode over the separator to the anode. The best liquid electrolytes (e.g. salt dissolved in water) have an electrochemical stability window at low voltage, whereas the lithium ion battery operates in a high voltage window. Thus electrolytes with an electrochemical stability window at high voltage are needed. Such electrolytes are Li salts (such as LiPF6 and LiBF4) dissolved in organic liquids solvents, and typical examples for the liquid solvents are linear or cyclic carbonates like propylene carbonate or di-ethylene carbonate. These electrolytes have a relatively low ionic transport rate. The transport rate is still much higher than those of the cathode materials but much less than for water based electrolytes. These facts illustrate that the ionic transport rate across the electrode is an issue in se. In a rechargeable Li battery the electrode thickness is determined by the liquid electrolyte properties. Without going into details—the relatively low ionic conductivity of the organic solvents and certain transport properties of binary electrolyte (electrolyte depletion) limits the thickness of the electrodes. If the current is too high or the electrodes are too thick, then a mechanism—called electrolyte shutdown—limits the capacity at high discharge rate. Electrolyte shutdown is a property related to the binary liquid electrolyte. An ionic transport within the solid cathode material is much slower, but the shut-down mechanism does not happen in the cathode material.
In order to achieve an acceptable rate performance Li ion batteries are made of electrodes which (1) contain enough porosity (to be filled with electrolyte in the final battery), and (2) need to be sufficiently thin (meaning low loading of active material (mg/cm2)) to allow a reasonable transport of lithium across the electrode. Typical porosities are >12 vol %, often 15 vol %, whereas loadings of 15-20 mg/cm2 are typical values. Porous, relatively thin electrodes are obtained by the relatively expensive ‘thick film technology’. As the ionic transport is much faster in the electrolyte than in the solid, there is a natural limit for increasing the density of the electrodes. If the porosity is too low then not enough electrolyte is present to support a sufficient fast ionic transport. Thus it would be highly desirable to develop a cathode material which has a high ionic transport rate so that some of the Li transport across the electrode happens via the solid particles. In this way higher current rates can be applied.
The thickness of the electrodes and the porosity could be lowered, which results in an increased energy density of the lithium battery, because more active material fits into the confined volume of the battery. Or the electrodes can be prepared thicker (but still support a high rate) and the porosity can be decreased. No cathode material has yet been reported whose ionic transport rate approaches those of liquid electrolytes.
Currently, due to a lack of safety of a charged battery, Li metal cannot be used as anode. In general, anodes which contain extractable lithium are dangerous to handle and are difficult to process. As a result the lithium needs to be supplied by the cathode, which potentially limits the choice of cathodes. The cathode typically is a lithium containing intercalation material. In intercalation materials lithium can be electrochemically reversibly extracted and reinserted. Presently only lithium transition metal oxides (or phosphates) are used as cathodes in rechargeable Li ion batteries. In the charged battery a delithiated transition metal oxide is in good contact with the organic electrolyte, as the latter fills the pores between the particles. If the battery becomes “unsafe” (for example by external damage or heating) then a chain of reactions can be triggered. A main reaction—very much determining the safety of the real battery—is the reaction between the delithiated cathode and the liquid electrolyte. This reaction is basically the combustion of the solvent with oxygen from the charged cathode. We will call it CCE (charged cathode-electrolyte) reaction within this invention. Batteries with less, or without organic electrolyte would potentially be much safer because no CCE reaction can happen. Such batteries are not available, because the rate performance of the battery is too low, as was discussed above.
Carbon based anodes have been widely applied in rechargeable lithium batteries. A typical charge capacity Qch (lithiation of the anode) is 360 mAh/g and a typical discharge capacity Qdc (delithiation of the anode) is 330 mAh/g. Thus a typical anode charge efficiency is 330/360=91.7%. It is convenient to consider the irreversible capacity instead: Qirr=1 minus charge efficiency, or Qirr=(Qch−Qdc)/Qch. A rechargeable lithium cell contains cathode and anode. The best utilization of the reversible capacities of anode and cathode—yielding a good cell balancing—is achieved if the charge efficiencies match. If not, an excess of cathode or anode material is needed, which excess does not contribute to the capacity of the lithium battery. Moreover the charge efficiency should be matched not only at slow charge/discharge, but also at fast discharge.
In the following discussion we focus on Li batteries with very high energy density. Very high energy density can be achieved by cathodes having either one or (preferably) both of a high volumetric density and a high specific reversible discharge capacity.
High volumetric density is easily obtained with relatively large, dense particles. LiCoO2 (LCO) is a very preferred material and can obtain high electrode density. This applies especially to LiCoO2 as described in WO2012-171780. LiNiO2 based materials also allow for relatively high density electrodes as well. Such particles can only be applied in a battery if the Li diffusion constant of the positive electrode is sufficiently high. If Li diffusion is too slow then the diffusion path within the particles needs to be shortened, which can be achieved by reducing size and increasing intra particle porosity, thus ultimately resulting in nano-structured (high surface area and meso porous) cathode materials. It is practically very difficult or even impossible to achieve high density with nano structured cathode materials.
A high specific capacity can be achieved with high lithium and manganese compositions—also called HLM, being Li—Mn—Ni—O2with Li:M>>1 and Mn:Ni>>1—cathode materials. They can be understood as a solid state solution of Li2MnO3 and LiMO2 where M=(Ni1/2Mn1/2)1−yCoxNiy. x>0 signifies Co doped HLM. These compounds are sometimes considered to be nano-composites. A strict distinction between the compounds is not possible because a nano-composite—as the composite size decreases towards atomic scale becomes a solid state solution. Undoped HLM cathode materials have a very high capacity—up to 290 mAh/g. The 290 mAh/g is typically achieved after several activation cycles at a voltage of 4.8V and discharge to 2.0V. These HLM cathode materials generally have very poor electronic conductivity and slow lithium diffusion, and therefore are prepared as nano-structured powders, making it very difficult to achieve a high electrode density. After activation undoped HLM cathodes need to be charged to high voltage (at least 4.5-4.6V) otherwise their capacities are not sufficiently high. At these high voltages, surprisingly, HLM can cycle in a stable manner with little capacity fading.
The cathode materials mentioned before—LiCoO2 (LCO) and HLM—are not matching the anode charge efficiency well. LiCoO2 as described in WO2012-171780 can have a very high charge efficiency of about 99%, even at high rate, which is much higher than that of typical anode materials. This high charge efficiency is also obtained with large particles having low surface area. Even these large particles show a high rate performance and very high charge efficiency, also at fast rate. Contrary to this, HLM has a low charge efficiency, which decreases dramatically if the rate is increased. Even submicron sized HLM cathode materials (with a D50 of 0.5-0.9 μm) with high surface area show poor rate performances and low charge efficiencies at fast rate.
Even if different materials exist that provide either high volumetric density or high specific capacity, there is the need to develop one material that has both characteristics, and at the same time has a high ionic conductivity and enables to operate in a rechargeable battery in a safe manner, and well balanced with the anode material.